1. Field of the Invention
This invention relates to a method for forming a thin film, particularly to a method for forming a thin film suitable for a secondary battery field usable for mobile electronic device and electric automobiles.
2. Related Art Statement
So far, a sol-gel method, a CVD method or a PVD method is employed as a thin film-forming method. These methods require a firing process after a molding process, a high vacuum condition, or a high energy condition accompanied with a substrate-heating process or a plasma-generating process. Therefore, those methods require large scale and complicate apparatus, resulting in large cost and complicate operationality in use.
Moreover, the above high energy condition runs counter to global environmental protection, resource saving and energy saving. Therefore, a new thin film-forming method without the above high energy condition has been desired.
It is an object of the present invention to provide a new thin film-forming method not including a high energy condition due to firing, heating or plasma generation.
This invention relates to a method for forming a thin film comprising the steps of:
setting a porous base material in between a pair of electrodes,
flowing a first reactive solution in between one electrode of the pair of electrodes and the base material,
flowing a second reactive solution in between the other electrode of the pair of electrodes and the base material, and
applying a given voltage between the pair of electrodes, thereby to synthesize a compound thin film including the components of the first reactive solution and the second reactive solution on the porous base material.
The inventors related to the present invention have been intensely studied for developing a new thin film-forming method not including a high energy process. As a result, they have found out surprisingly that when a porous base material is set in between an anode electrode and a cathode electrode to which a given voltage is applied and a first reactive solution and a second reactive solution, which are different each other, are flown in between the cathode electrode and the base material and in between the anode electrode and the base material at their predetermined flow rates, respectively, a compound thin film including the components of the first reactive solution and the second reactive solution is directly synthesized on the base material.
FIG. 1 is a conceptual view showing the state in which the compound thin film is synthesized directly on the porous base material.
In this case, the first reactive solution is made of a water solution with a melted LiOH.H2O in a distilled water, and the second reactive solution is made of a water solution with a melted CoSO4.7H2O in a distilled water.
The first reactive solution is flown in between a cathode electrode 1 and a porous base material 3 at its given flow rate, and the second reactive solution is flown in between an anode electrode 2 and the porous base material 3 at its given flow rate. When a given voltage is applied between the cathode electrode 1 and the anode electrode 2, the first and the second reactive solutions are dissociated, and thus, Li+ ion particles exist in between the cathode electrode 1 and the porous base material 3 and Co3+ ion particles exist in between the anode electrode 2 and the porous base material 3.
Then, these ion particles arrive at the porous base material, and the Li+ ion particles and the Co3+ion particles pass through the surface inter-connecting holes of the base material 3 and arrive at the opposite surfaces 3B and 3A thereof, respectively. The Li+ ion particles on the surface 3B react with a large amount of Co3+ ion particles and oxide elements in the water solution to form a Co-based oxide thin film, made of LiCoO2 or the like, on the surface 3B of the porous base material 3.
Moreover, the Co3+ ion particles on the surface 3A react with a large amount of Li+ ion particles and oxide elements in the water solution to form a Co-based oxide thin film, made of LiCoO2 or the like, on the surface 3A of the porous base material 3. As a result, a compound oxide thin film such as the Co-based oxide thin film is directly synthesized and stabilized on the porous base material.
In this case, if the difference in the flow rates and/or the pressures between the first and the second reactive solutions is controlled appropriately, the compound oxide thin film may be formed on either of the surfaces 3A and 3B or in the porous base material 3.
In the thin film-forming method of the present invention, since the reactive solutions are flown constantly, the component particles such as the Co3+ ion particles and the Li+ ion particles to constitute the compound thin film always exist at their constant ratio. Therefore, the compound thin film can be formed uniformly, and a particulate compound or a powdery compound is not formed.
According to the thin film-forming method of the present invention, the crystalline compound thin film can be formed on the porous base material without a firing process after a thin film-forming process and a high energy condition including a substrate-heating process and a plasma generation